Marshall JCM800 Emulation - no signal

[ylovits]

Hello everyone.

I tryied to make a Marshall JCM800 Emulation (link)

but failed... unfortunately
Everything is in place. Nothing is touching on the back side of the veroboard.



But! i used a keyswitch (just for fun) witch might have shorted with the enclosure
this is just an idea, i dont know if this is the real problem...
i removed the keyswitch and the problem remains..




Problem description:
I have signal when the pedal is bypassed, but when the pedal is engaged there is no signal.
The led lights up.

I know i might have to make the board from the beginning, but i would love if there were any ideas how i can avoid it.

thanks

[alltrax74]

Did you plug the trannies into the sockets ?
This seems to be a silly question but many of us spent time debugging while the active parts were not in the circuit...
Otherwise check for component misplacing, solder bridge, bat trace cut...

[ylovits]

Thanks mate. but i plugged the trans.
I checked the parts for misplacement
I looked for solder bridges
I tried also the circuit out of the encloseure.
Nothing of those.

what is "bat trace cut"?

[alltrax74]

Typo, bad trace cut underneath the board. A bad cut can let the signal pass through

[ylovits]

Aaaa. Got it. No... No bad cut either. 😢

On Sep 7, 2017 6:12 PM, "alltrax74 [via Guitar FX Layouts]" <[hidden email]> wrote:

Typo, bad trace cut underneath the board. A bad cut can let the signal pass through

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[ylovits]

I replaced the trans and i biased them very carefully and some signal comes out.
The pots function as expected but the overall signal if thin and weak...
any ideas?

[mathbrook]

Audio probe, use the schematic.

[ylovits]

I used an audio probe. But i dont clearly understand the schematic. I know very little about what si going on on the circuit.

I can tell these results fron the audio probing:

First Image) on the green circles i can hear the sgnal pass through loud and clear, on the red circle i hear the signal distorted and weak.



Second image) If i short with aligator clips as shown in the picture i get a nice loud signal ont the green circle



Third image) i didnt get any signal on the red circle. i dont know if this is normal...



does any of the information help to identify the issue?

[alltrax74]

This is not the right way to do.
You have to use the schematic, not the layout.
You plug your guitar into the probe and start from the end of the schematic. At the output it's like if you bypass the pedal so you will have the sound of your guitar straight to the amp.
Then you move your probe following the schematic from the end to the input.
When you loose the signal or hear something weird, you know where your problem is and try to figure out what's wrong at this location on your build

[ylovits]

do you have a schematic?

[Hozy31]

Looking at your picture of the board, the bottom leg of the top 1k looks as though light is shining through which would suggest a missed or dodgy solder. Could be a trick of the light though.

[ylovits]

I made it again from start.. these photos are from the first try

[alltrax74]

No, I didn't even build it, but it should be similar to this one, at least it's the same architecture https://postimg.org/image/bb807av67/
With this schematic and the layout (and some thinking) you should be able to probe it.
Debugging is not the funniest part but is certainly where you learn the most

[ylovits]

I did as you described using this schematic (it is the same as the layout without the 100nF cap on the output)



i found out that the problem is on the Q5, the signal is normal at the gate, and weak and distorted at the Drain

Although the voltage on the Drain is setup
I swaped the Q5 with a brand new but it was not this component's fault.

followed all this line:



Yet i dont understand what i should do with this information :P
why whould the signal be distorted at the Q5 drain?

[ylovits]

The other thing i noticed, is that when i connect the Q5 to my body the signal gets better.
But if i connect it to ground the signal dissapears

Maybe this helps figure out whats goin on

[alltrax74]

It's hard to tell without the board.
You have located the issue (at least one of them, they could be multiple)
Check below the board for bad trace cut, solder bridge
Check for component misplacement, wrong value.

You could also do the opposit, plug the probe to the amp and start probing from the circuit input towards the output to see if everything is ok until Q5

[ylovits]

I got to Q5 the other way around, and everything was ok.
I will replace all capacitors around it and then all resistors cause i feel there is a problem with the grounding of Q5's source.
thanks for all the help
hope this time i make it... or else it is time to dispose it :(

[ylovits]

I let go :(
maybe next time :)